1. Field of the Invention
The present invention relates to optical pulse testers, and more particularly, to optical pulse testers which inject optical pulses into an optical fiber and demultiplexes returning light from the optical fiber into Raman scattered light, and which displays the Raman scattered light.
2. Background Art
An example of the structure of a conventional optical pulse tester will be explained with reference to FIG. 8. As shown in FIG. 3, an optical pulse generator 1, a directional coupler 2, a light receiver 8, an amplifier 4, a display 5, and an optical fiber 10, are provided. In FIG. 8, the optical pulse generator 1 emits optical pulse 11 with wavelength .lambda.a. The optical fiber 10 is to be measured and comprises an optical fiber 10B connected to an optical fiber 10A with an connector (not shown).
The optical pulse 11 with wavelength .lambda.a from the optical pulse generator 1 is injected into the optical fiber 10 via the directional coupler 2. The optical pulse 11 travels through the optical fiber 10 with the generation of Rayleigh scattered light 14 and Raman scattered light (not shown). Furthermore, Fresnel reflection light 18 is generated at the connecting point of the connector and the like. The parts of the scattered light generated in the optical fiber 10 return to an injected end of the optical fiber 10 as back-scattered light, that is, returning light 12. The returning light 12 from the optical fiber 10 is divided into the Fresnel scattered light 13 and the Rayleigh scattered light 14 in the directional coupler 2 and the Fresnel scattered light 13 and the Rayleigh scattered light 14 are received by the light receiver 3. The Fresnel scattered light 13 and the Rayleigh scattered light 14 in the returning light 12 are detected by the light receiver 3. The outpost light from the light receiver 3 is amplified by the amplifier 4. The output light from the amplifier 4 is displayed on the display 5.
Next, an example of the displayed waveform on the display 5 shown in FIG. 3 will be explained with reference to FIG. 4. FIG. 4 shows an example of the characteristic waveform of the optical fibers 10A and 10B in the case of connecting the optical fiber 10B to the optical fiber 10A with the connector. In FIG. 4, a vertical axis indicates loss of the optical fibers 10A and 10B and a horizontal axis indicates distance from the injected end of the optical fiber 10A. The vertical axis of FIG. 4, for example, indicates 40 dB with full scale. In FIG. 4, a peak 13A indicates the Fresnel reflection light 13 at the injected end of the optical fiber 10A. A straight line 14A indicates the Rayleigh scattered light 14 in the optical fiber 10A decreases exponentially with distance from the injected end of the optical fiber 10A (to the right of the horizontal axis in FIG. 4) and is logarithmically transformed. In FIG. 4, a peak 13B indicates the Fresnel reflection light 13 at the connecting point where the optical fiber 10B is connected to the optical fiber 10A. A straight line 14B indicates the Rayleigh scattered light 14 in the optical fiber 10B decreases exponentially with distance from the injected end of the optical fiber 10A and is logarithmically transformed. A peak 13C indicates the Fresnel reflection light 13 at an outgoing end of the optical fiber 10B. In FIG. 4, the right side area of the peak 13C indicates noise.
In FIG. 4, the Fresnel reflection light 13A, 13B, and 13C and the Rayleigh scattered light 14A and 14B have the same wavelengths as the optical pulse 11. Accordingly, when the Rayleigh scattered light 14A and 14B are measured, since the Fresnel reflection light is incomparably larger than the Rayleigh scattered light, the amplifier 4 shown in FIG. 3 may be supersaturated by the Fresnel reflection light 13A, 13B, and 13C. When the amplifier 4 becomes supersaturated by the Fresnel reflection light 13A, 13B, and 13C, a dead area is generated in the amplifier 4 until a circuit of the amplifier 4 becomes stable. In the dead area, the state of the optical fiber 10A and 10B are not clear for a constant time. In FIG. 4, numerical reference 21 and 22 indicate the dead area caused by the Fresnel reflection light 13A and 13B, respectively. In the part of the dead area 21 and 22, the state of the near-end part of the optical fiber 10A and 10B cannot be exactly measured. An optical switch operating at high speed may be inserted between the directional coupler 2 and the light receiver 3 shown in FIG. 3 so as not to inject the Fresnel reflection light 13 in the light receiver 3, so that it is possible not to supersaturate the input light in the amplifier 4. However, since the switching speed of the optical switch and the time corresponding to pulse width cannot be measured, the dead area cannot be eliminated. Accordingly, in the conventional art, a short pulse is used as the optical pulse 11 or the response speed of the circuit is improved so as to reduce the dead area.